This invention relates to an interferometer, and in particular to an interferometer which utilizes white light.
Optical interferometer arrangements have been used for a long time for the point-by-point or the large-area determination of surface topographies or layer thicknesses, as they permit highly accurate, non-destructive and fast measurements. Examples of such arrangements are the interferometers according to Michelson or Mach-Zehnder. The known interferometers are generally operated with monochromatic light, so that the resultant interference fringe pattern may be more readily evaluated. The evaluation accuracy of such interference images is of the order of fractions of half a wavelength (the spacing of two adjacent interferometer fringes) and may be increased by electronic means to a value of .lambda./100. A typical disadvantage of interferometers operating with monochromatic light is that only the phase differences of interfering waves corresponding to a maximum path difference of .lambda./2 may be uniquely measured, since larger phase differences (i.e. larger vertical deviations or changes in layer thickness) result in the same interference pattern. Apart from this, surface structures with hills and hollows look identical in monochromatic interference images.
For overcoming those disadvantages, several modifications of monochromatic interferometers are known from the art, e.g., using obliquely incident light to increase the unique measuring range, periodic movements to distinguish between hills and hollows, etc.
Another typical concept of providing unique interferometric measurements consists in the use of several wavelengths or of white light in lieu of monochromatic light. In that case, there are colored fringe patterns, e.g. the known Newton rings, instead of the known black-white fringes or bands. Although the evaluation of these colored interference phenomena (according to color and amplitude) permits in principle unique measurements of height profiles or layer thicknesses, it is elaborate when applied individually and therefore has only been used on a limited scale.
An example of evaluating colored interference fringes for layer thickness measurements is described by S. Tolansky in the book "Multiple-Beam Interference Microscopy of Metals", London/New York 1970, p. 141 et seq. According to this example, the colored interference fringes are spectrally analyzed, determining the layer thickness and the layer gradients (ascending or descending portion of a surface) from the spacing of the dark interference fringes, the so-called Mueller fringes, appearing in the spectrum. However, this method is only suitable for analyzing line-shaped surface sections but not the entire surface, an analysis of which would be highly time-consuming.